Sensors for measuring blood components have been conventionally used in, e.g., clinical examinations or self-monitoring of blood glucose levels by diabetics. The sensors for measuring blood components may have a configuration in which a cover is disposed on an insulating substrate via a spacer. A working electrode and a counter electrode are formed on the surface of the insulating substrate. A reagent containing an oxidoreductase, a mediator (electron carrier), etc. is placed on the working electrode and the counter electrode. This portion is an analytical portion. One end of a flow path for the introduction of blood is connected to the analytical portion. The other end of the flow path is open to the outside and serves as a blood inlet. Analysis of blood components (e.g., blood glucose levels) using such a sensor may be performed in the following manner. First, the sensor is set in a special-purpose measuring device (meter). Then, a lancet is used to prick the finger tip or the like and draw a small amount of blood, which is brought into contact with the blood inlet of the sensor. The blood is sucked into the flow path of the sensor by capillary action, passes through the flow path to the analytical portion, and comes into contact with the reagent. An oxidation-reduction reaction occurs between components of the blood and the oxidoreductase, so that a current flows via the mediator. After the current is detected, the measuring device calculates a blood component amount based on the detected current value and displays the blood component amount.
In this manner, the blood component amount can be measured using the sensor. However, the measured value may be affected by hematocrit (Hct). Therefore, in order to obtain an accurately measured value, it is necessary to measure an Hct value and to correct the blood component amount based on the Hct value. For example, there is a sensor that includes two working electrodes and one reference electrode to measure an Hct value and corrects the blood component amount with the Hct value (see Patent Document 1). Moreover, there is a method that uses a mediator to measure an Hct value (see Patent Document 2). However, the conventional techniques have problems in the accuracy and reliability of the measured Hct value, and thus cannot sufficiently and properly correct the blood component amount.